116 DEVELOPMENT OF THE THORACIC DUCT 



Fig. 171 shows a reconstruction of the left vascular complex of 

 the lower cervical and upper thoracic regions in a 15.5 mm. cat 

 embryo (series 143, slides viii, ix, x, xi, and xii, 225) viewed 

 from the right side and from behind, and figs. 172 to 183 

 give transverse sections of the left upper thoracic region in the 

 same embryo, selected from slides xi and x. The figure of the 

 reconstruction gives by means of side lines and roman numerals 

 the level of the slides entering into its composition, so that the 

 individual sections shown in the microphotographs can be referred 

 to and oriented by the reconstruction. 



The reconstruction as well as the photographs of the sections 

 should be followed from below upwards. 



Description of reconstruction, fig. 171. The reconstruction is 

 carried caudad to the upper azygos region. The right azygos 

 arch (3'} is seen entering the dorsal circumference of the right 

 precava (3). Azygos (6') and precava (6) of the left side cannot 

 be distinctly seen in this view of the model. Mesal to the right 

 praecava (3) is the right vagus (22}. 



Projecting cephalad above the azygos arches and the interazy- 

 gos anastomosis are seen the cephalic elements of the azygos por- 

 tion of the thoracic ducts (36}, which extend thence caudad along 

 the ventral surface of the azygos complex. The cephalic end of 

 the azygos segment of the ducts has not yet joined the caudal 

 extremity of the preazygos segment (35}, although they have 

 approached each other closely, and adjacent detached lymphatic 

 elements between them foreshadow their coming union. 



The preazygos segment of the thoracic duct (35} is a well devel- 

 oped plexiform channel, w r hich receives a considerable branch 

 from the lymphatic complex along the ventral aspect of the pre- 

 vertebral venous plexus (17}. The latter is now fully formed and 

 differentiated clearly from the adjacent lymphatics, draining as 

 a chain of venous spaces caudad into the azygos arches. This 

 central part of the preazygos duct (35} intersects with the medial 

 aspect of the large vertebral vessels (41, 4%) an d the underlying 

 left subclavian artery and receives just caudal to this point a 

 lymphatic channel connecting it with the broncho-mediastinal 

 duct. This lymphatic vessel cannot be seen distinctly in this 



